The rear Cab dome light no longer works no matter what position it is set (ON or Door). The bulb is good, there is no power to the light. I checked what fuses I could find and they are all good. I'm stumped on this one. I even disconnected the battery thinking it was a reset that was needed - no good.

Welcome to TW. The 15A dome fuse supplies the power to the body ecu. The power is then distributed to the dome switch via the body ecu. There should be a black wire in the dome switch that has the 12V on it. This wire also supplies the power to the personal lights (map lights). If the personal lights work and the dome light doesn't, then it is in the switch. I know on my 98, I had to have the switch just off the dome setting in order for it to work.

Well this has me flummoxed ... Map lights work just fine - and the switch to the cab light is good too ... just no voltage at the cab light. I am going to jump the wire from the ECU to the light to see if the light works that way.

Well this has me flummoxed ... Map lights work just fine - and the switch to the cab light is good too ... just no voltage at the cab light. I am going to jump the wire from the ECU to the light to see if the light works that way.

No power at the switch. I took the switch apart and jumped the connection with my voltmeter to verify power. No power. Voltage = zero. As far as I can tell, there is no power to the switch being provided.

Just in case, I took the light bulb out and powered it by the battery - came on like a charm.

Welcome to TW. The 15A dome fuse supplies the power to the body ecu. The power is then distributed to the dome switch via the body ecu. There should be a black wire in the dome switch that has the 12V on it. This wire also supplies the power to the personal lights (map lights). If the personal lights work and the dome light doesn't, then it is in the switch. I know on my 98, I had to have the switch just off the dome setting in order for it to work.

Hi, Okay so looks like you have a 15 amp fuse in the fuse box specifically for the dome lamp. From the fuse box there is a blue with yellow tracer wire to the body ecu.

Then comes out of the body ecu with a black wire which most likely runs up the driver side pillar. There is a connector before the wire gets up to the dome and map lights, but they both share the same wire so if one works and one does not then I'd say the connector is okay.

That black wire connects to the dome lamp and also supplies power over to the personal light (factory speak for map). The schematic at least does not show a connector or anything where the black wire splits off to go to the map lights. Only the central one that would effect both lamps power source.

The black wire is the power wire.

This is confirmed by the map light circuit that uses the power in from the dome/map light black wire and just grounds to the body there at the mount for the map light to complete the circuit.

The dome lamp switch when placed to the On position actually shows to ground from the lamp mounting itself (so body ground right there by being screwed in).

The Door position shows that it connects to a black with white tracer wire that goes through a connector and then back to the body ECU. This wire will supply a ground signal through the body ECU from the door switches.

Okay, so basics.

Check fuse to be sure it is good as there are other "power in" fuses connected to the body ecu.

Fuse good then pull the dome lamp, unplug and look for power on the black wire itself. You should have power as the ON position showing for the dome is straight to body ground so you should get something at the wire itself (don't try through the dome light body go to the actual wire).

If you have no power and verified the dome will not come on when connected (remember, has to be actually screwed into the roof so that it can ground) when switched to the ON position then look to the map light.

If the map light has power and works then there is a problem on the black wire somewhere between the dome lamp and where it branches off to the map lamp. Bad connection, a break in the wire or a short (although you'd think it would pop the fuse with a short).

No indication of where that is and I don't have experience with where, but if I had to guess I'd look towards right at the dome lamp or at the upper pillar area.

If you find you do have power on the actual wire at the dome lamp then I'd look to a faulty dome lamp. Especially if it does not come on in the ON position. Of course, verifying that the lamp body is getting a good clean ground when reinstalled to test.

Update: I checked the voltage at the lamp and it measured 0.5 volts on both "on" and "door" (door open) position. I guess I have to find that black wire and check how it is fed along with ground. Right now I don't see any wires in there and will probably have to figure out how to pull the light fixture out of the roof to expose the connections. Everything looks brand new under the lamp cover.

FYI, there are 2 power connectors going to the dome light, one has constant power, the other is connected to your door switch... If you unplug the connector from the dome light (after unscrewing from the cab), try using a multimeter to check the DC current between the metal roof of the truck and each of the connections on the plug that you disconnected... But try my above post first... worked for me...

mjz: Another long proceedure for you, but hope it helps.
Like scotrinaf and I both mention, you need to take the dome lamp itself out and get to the actual wires. This is where you need to test. The wires themselves. Then you will know where you actually are.

Shows that to remove your dome lamp, pry the cover off and remove the two screws that screw straight up into the roof to hold the lamp in.

I covered most of what to do above, but additional info below.

While you have the lamp out testing for power on the solid black wire, clean the ground points/scuff them down to get good contact. Also make sure that the wiring gets a good clean connection when it is installed into the dome lamp.

If you have power to the solid black wire and the dome lamp will not come on in the ON position when reinstalled, then I'd say the dome lamp either has a poor ground to the body still or there is a problem with the dome lamp itself. The lamp shows to ground through the lamp itself to the body in the ON position.

Okay, so lets say you actually pull the dome lamp assembly out, test the actual solid black wire itself, find it has power, clean and make sure the dome has a good ground and when reinstalled you finally get a light in the ON position, BUT you get no lamp with the switch in the door position and the doors open.

Then you should be looking to the black with white tracer wire I previously mentioned. Remove the dome lamp assembly and test for continuity to ground on this wire when the doors are opened. You should have continuity with the doors open and no continuity with the doors closed. I say doors as it would be highly unlikely that all the door switches would be bad so at least one should provide a ground point to work the system although I guess stranger things have happened. No dig in the least to scotrinaf here just going through the process and I guess could not hurt to manually work the switches and wiggle them to ensure they are making contact.

If you have continuity to ground on the actual black with white tracer wire at the dome location with the doors open then I'd look back to there being a problem with the dome lamp itself.

If you do not have continuity, then the black with white tracer wire goes through a connector (I'll have to look for location for you to find where although my guess would be behind one of the kick panels) and then to the body ECU. From the body ECU it connects to the door switches.

The door switches supply a ground point. Basically with the door closed the door pushes the spring switch inwards which releases their contact with ground to the body. This causes the dome lamp (in the door position on the dome lamp switch) to loose ground to the circuit and as such the lamp goes off. When the door is opened then the switch springs back out and makes contact with the body again creating a ground which completes the circuit and turns the dome lamp on again.

You want to check for a break in the black with white tracer wire, but more often than not the issue is with a poor connection at a connector. Have you made sure the body ECU plug is clean and tight?

Hillingdoner - Thanks for the summary. I am about to head outside and get to work on this.

I do believe it is a grounding issue. I do have 0.5 volt present when the switch is "on" and zero volt when I set the switch to off. The same happens with the doors open and closed and the switch set to door. That is telling me I have a connection problem (high resistance to the ground/roof metal). I have read elsewhere that this is a common problem with the cab light - so seems reasonable.

I tried the easy stuff first - hoping it was nothing more than a burned out light.

I'll post here what I find out. Hopefully it is just as you suggested.

I have the dome/cab light unit removed. I can see the black wire and the black/white stripe wire. When I test the voltage against ground, I get 0.5 volts.
The door voltage goes to zero when I close the door and rises to 0.5 volts when I open the door. Makes no sense. The map lights work just fine - so something is wrong between the map light and the dome/cab light ... and it is both wires - which makes even less sense (one wire would be reasonable).

I am going to take the map light apart to trace the wires back to that point. It's not at the fuse connection (under the hood). I don't know why the manual shows the wiring going through the body ECU - there's not a connection there. When I disconnected the wiring harness at that location (just to the left of the steering wheel by the hood latch) the map lights still came on. So the wires do not connect through there.

What a simple problem turning into a major headache this has turned out to be.

but I got it to work - sort of.
The switch is bad. I took the assembly out and put it on my bench with a 12V battery I have. The bulb worked fine so I wanted to test if it worked with the switch. Nothing. After cleaning it, I was able to get the bulb to light up nice and bright connected in the switch. Both sides work (ON & Door).

I put the assembly back into the truck and nothing. But after jiggling the switch - it lit up. It's not a solution because it won't stay lit ... but like you say, I had enough of this. At least I know what was wrong and someday I'll replace the part. In the mean time, when I need light in the cab, I can get it to light.